FORMATION OF LARGE LINEAR TROUGHS AND STREAMLINED RIDGES BY DIRECT DEFLATION OF PEORIA LOESS, NEBRASKA, USA

Large linear troughs and ridges up to 20 km long are found in thick Peoria Loess of Nebraska. We hypothesize that these features were generated by large-scale erosion of loess immediately following loess deposition in the late Pleistocene. The ridges and troughs are oriented northwest-southeast, consistent with Pleistocene wind directions inferred from trends in thickness and grain size. We used the Portable in-situ Wind Erosion Lab (PI-SWERL) to measure the potential of loess to be entrained by the wind. PI-SWERL tests were conducted on benches that were carved into in-situ loess at field locations. Coarse to fine loess was also collected and tested in the laboratory setting. PI-SWERL tests were conducted on 1) dry, loose loess, and 2) crusted loess. Results indicate that most dust generation, in the field and lab, occurs by direct deflation of dust. Saltation, even in coarse loess, was nonexistent to intermittent. In a few cases, saltation of soil aggregates preceded dust generation or enhanced emissions after direct entrainment began. Grain-size analysis reveals the loess has a significant aggregate population. Emission fluxes of loess are comparable to other regional dust-producing landforms. Magnitude of dust formation is large in both proximal and distal settings, suggesting that loess was easily entrainable provided vegetation density was much lower than present. Crusting is effective at minimizing emissions, especially in fine-grained loess. Spatially variable erosion of loess after deposition is compatible with grain-size data. Thick loess on high ridges between troughs fines systematically toward the southeast, consistent with remobilization. At one site, a northwest-southeast swath of very coarse loess at intermediate elevation indicates remobilization and winnowing of fines from loess after initial deposition, possibly representing incipient trough development. Thinner loess on trough floors displays large variations in grain size suggesting episodic remobilization.